In wireless communications networks, downlink signals are encoded using forward error correcting codes to allow correction of bit errors by the receiving network equipment. Two common types of forward error correction encoding used in long term evolution (LTE) and LTE advanced networks are convolutional encoding and Turbo encoding. A convolutional decoder receives downlink signals and uses the error correcting code to identify the most likely combination of transmitted bits based on the received bits.
Interference in the received signal can lead to bit errors. One possible cause of interference in downlink signals occurs in networks that support both legacy LTE and LTE advanced user equipment. LTE advanced is supposed to be backwards compatible with legacy LTE equipment. However, LTE advanced equipment introduces new reference signals for the purposes of Channel State Information (CSI) and positioning. These reference signals are transmitted in particular resource elements in particular subframes depending on configuration. LTE advanced user equipment (UE) understands and properly decodes these reference signals. However, legacy UEs will experience interference due to the reference signals introduced on LTE advanced because different antenna ports of the e-Node B transmitting data and reference signals to the legacy UEs which may use the same resource elements used by other antennas for the reference signals. When the same resource element is used for a reference signal and data is received by a legacy UE, the presence of the reference signal may cause interference and possibly bit errors in the decoding of the data by the legacy UE because the legacy UE has no knowledge of the reference signal or the data that was transmitted.
One possible solution to the problem of interference caused by reference signals is to lower the modulation and coding scheme (MCS) index, which lowers the number of bits transmitted per symbol and thus lowers the data rate. For example, the MCS index may be changed from MCS 23, which corresponds to 64 quadrature amplitude modulation (QAM) encoding, to MCS 12, which corresponds to 16 QAM encoding. Lowering the MCS index decreases the likelihood of bit errors because lower coding and the space between symbols in the signal constellation is increased. However, lowering the MCS index decreases the data throughput provided to the legacy UEs. Lowering the throughput data throughput is undesirable as it decreases the quality of service experienced by users.
In addition to being a problem for legacy UEs that operate in networks with advanced network equipment, interference caused by the reference signals is also a problem for network equipment test systems that emulate legacy and advanced network equipment. It is often desirable for UE emulators to emulate as many UEs as possible to test the performance of access network equipment, such as evolved node Bs (e-Node Bs). If the coding rate is slowed because of bit errors experienced by emulated legacy UEs caused by interference from reference signals transmitted to real or emulated LTE advanced user equipment, the number of UEs that can be emulated and the total system throughput may be adversely affected. Even if a single legacy LTE UE is being emulated in a test, the data rate to that UE can be slowed if bit errors caused by interference from LTE advanced reference signals occur.
Accordingly, in light of these difficulties, there exists a need for methods, systems, and computer readable media for smart decoding of downlink signals in the presence of interference caused by reference signals of different generation network equipment.